System Tray New Mail Icon (Outlook 2000)

Has anyone ever run into a problem of extra "new mail" icons in the System Tray? Occasionally, when I click on a new message the mail icon in the System Tray doesn't disappear. Then when a new message arrives, another envelope appears in the tray. Shutting down Outlook doesn't seem to clear the icons; I have to re-start the computer to get rid of them.
I assume there must be a simple solution to this problem. <img src=/S/confused.gif border=0 alt=confused width=15 height=20>